| 摘要 |
Temperature lowering and stain resistance of a spark plug for internal combustion engine are enhanced. The spark plug (1) has an insulator (2), a main body metal fitting (3), a central electrode (5), and an earth electrode (27), and a spark discharge gap (33) is formed between the tip end (28) of the central electrode (5) and the earth electrode (27). A through hole (29) is formed in the main metal (3) and a metal fitting protrusion (21) that swells radially inward is formed in the through hole (29). The metal protrusion (21) has a protrusion rear side face (30), a protrusion inner circumferential surface (31), and a protrusion tip side face (32) wherein the inner circumferential surface (40) of the through hole (29) closer to the tip side than the protrusion tip side face (32) has a fixed inside diameter A (mm). The insulator (2) is inserted into the through hole (29) and has first and second insulator tapered portions (14, 36), and a base portion (37) between the tapered portions. When assuming the outside diameter of a border portion (K) between the second insulator tapered portion (36) and the tip end (38) of the insulator is B (mm), the distance of spark discharge gap (33) is G (mm), and the length from the forefront part (FF) of the protrusion inner circumferential surface (31) to the border portion (K) in the axial C1 direction is XX (mm), the following relations are satisfied; G<=(A-B)/2, A>=7.3, and 2<=XX<=4. |
